Social Icons

Pages

Saturday, 27 April 2013

THE REACTION BETWEEN METHANE AND BROMINE

A Free Radical Substitution Reaction
This page gives you the facts and a simple, uncluttered mechanism for
the free radical substitution reaction between methane and bromine. If
you want the mechanism explained to you in detail, there is a link at
the bottom of the page.The facts
This reaction between methane and bromine happens in the presence of
ultraviolet light - typically sunlight. This is a good example of a photochemical reaction - a reaction brought about by light.

Note: These reactions are sometimes described as examples of photocatalysis
- reactions catalysed by light. It is better to use the term
"photochemical" and keep the keep the word "catalysis" for reactions
speeded up by actual substances rather than light.

CH4 + Br2CH3Br + HBr

The organic product is bromomethane.
One of the hydrogen atoms in the methane has been replaced by a
bromine atom, so this is a substitution reaction. However, the reaction
doesn't stop there, and all the hydrogens in the methane can in turn be
replaced by bromine atoms. Multiple substitution is dealt with on a
separate page, and you will find a link to that at the bottom of this
page.

Warning! Check your syllabus at this point. If your
syllabus wants you to know about the free radical substitution reaction
between methane and chlorine as well as this one, don't waste time
trying to learn both mechanisms.
The two mechanisms are identical. You just need to learn one of
them. If you are asked for the other one, all you need to do is to write
bromine, say, instead of chlorine.In
writing the bromine mechanisms on these pages, that's exactly what I've
done! If you read both chlorine and bromine versions, you'll find them
boringly repetitive!

The mechanism
The mechanism involves a chain reaction. During a chain
reaction, for every reactive species you start off with, a new one is
generated at the end - and this keeps the process going.

Species: a useful word which is used in chemistry to
mean any sort of particle you want it to mean. It covers molecules,
ions, atoms, or (in this case) free radicals.

The over-all process is known as free radical substitution, or as a free radical chain reaction.

Note: If you aren't sure about the words free radical or substitution, read the page What is free radical substitution? Use the BACK button on your browser to return quickly to this page.